[squarefoot]

"So, have the bottom cover be the heatsink?"

Yes, the upper cover with the fins up should be designed to match perfectly with the lower one when mounted with the fins facing down. Cuts should be arranged so that the bottom cover rubber feet wouldn't prevent perfect surface contact. It would become a fairly large heatsink in which the size and combined thickness would likely be enough to counteract the small fins size and absence of fans. Battery/disk/memory covers on the bottom side would be accessible by removing the additional cover.